Low-cost single-fiber multi-directional transceivers such as diplexers and triplexers used in Optical Network Unit (ONU) are of significant importance to the development of Fiber-To-The-Home (FTTH) networks. Traditional diplexers and triplexers are based on thin-film-filter (TFF) technology that uses lenses, filters, and other micro-optic components assembled by hand. Although the manufacturing cost has been reduced significantly in recent years, further cost reduction is difficult due to the labor-intensive packaging. A monolithic integrated solution that reduces the number of subcomponents and alignment emerges as a promising alternative. It is especially suited to mass production which can meet the demand of the potentially explosive growth of the FTTH market. However, monolithic integration needs expensive InP-compatible materials to build all components of different functionalities. Diplexers/triplexers with large dimensions and complex fabrication will greatly increase the cost, and counteract the advantages brought by the monolithic integration.
At an ONU, triplexers are used to separate two downstream signals transmitted in a fiber at two different wavelengths and direct them to a digital signal receiver and an analog signal receiver while coupling an upstream optical signal emitted by a digital transmitter to the same fiber. They are essential components for FTTH systems. Usually the downstream digital signal received by the ONU is at 1490 nm, analog signal at 1550 nm, and the upstream digital signal at 1310 nm. In some systems, the 1550 nm analog channel is not used. Diplexers are then used to receive and transmit digital signals at 1490 nm and 1310 nm, respectively.
An InP-based single-fiber bidirectional transceiver was reported in an article entitled “Monolithically Integrated Diplexer Chip for PON Applications”, by A. Behfar et al., in proceedings of the Optical Fiber Communication Conference, vol. 2, pp. 3, 2005, as showed in FIG. 2. An InGaAlAs based multiple quantum well laser structure is grown on top of an InGaAs p-i-n structure by metal organic chemical vapor deposition on an InP substrate. A Fabry-Perot Horizontal Cavity Surface Emitting Laser which contains a 45 degree etched mirror is used as the upstream signal transmitter. The 1310 nm light generated in the laser cavity is reflected out of the plane of the chip by the 45 degree etched mirror and focused by a lens to an optical fiber while 1490 nm downstream beam launched from the fiber is broaden by the lens, projected onto the chip and finally absorbed by the PIN detector. Although the chip provides a low cost integrated solution of diplexers, it has a significant drawback due to the fact that a lens is required for coupling the upstream/downstream signals from and into the fiber. The use of bulk optical components (e.g. lens) complicates the package processes. Beside, it is difficult to extend it for use as a triplexer due to its vertically coupled multiplexing mechanism.
A simple diplexer can be realized using an extra-cavity 1×2 or 2×2 coupler that couples the upstream and downstream signals to an optical fiber as shown in FIG. 3. The downstream signal launched into port 1 is coupled to port 2 and then absorbed by the following photodetector, while the upstream signal emitted by the laser 6 is coupled to port 1. It can be realized by hybrid integration or monolithic integration. However, a disadvantage of this simple diplexer is that a part of the downstream signal is coupled to port 3, which will not only introduce downstream loss but also disturb the transmitted laser signal from port 3. Similarly, a portion of the upstream signal is unintentionally coupled to port 4, causing an upstream loss. There is a compromise between the upstream and downstream losses in this design, i.e. if the downstream loss is low, the upstream loss is high. Usually a 3 dB coupler is used which produces a 50% loss for each signal.
A more complex approach for realizing diplexer/triplexer is to use a wavelength multiplexer such as an Arrayed Waveguide Gratings (AWG), an Echelle Diffraction Gratings (EDG), or a Mach-Zehnder Interferometers (MZI) to accomplish highly wavelength selective coupling. Such an element, however, has disadvantages of large foot prints, complex fabrication processes, and high cost.